Drilling fluids used in the drilling of subterranean oil and gas wells as well as other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”. The drilling fluid is expected to carry cuttings up from beneath the bit, transport them up the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and also often to form a thin, low permeability filter cake which temporarily seals pores, other openings and formations penetrated by the bit. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that herein, the term “drilling fluid” also encompasses “drill-in fluids”.
Drilling fluids are typically classified according to their base material. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water or brine. Nonetheless, the water is the continuous phase. Oil-based muds are the opposite. Solid particles are suspended in oil and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds that are water-in-oil emulsions are also called invert emulsions. Brine-based drilling fluids, of course are a water-based mud in which the aqueous component is brine.
Optimizing high performance water-base mud design is commonly at the forefront of many drilling fluid service and oil operating companies' needs due to the various limitations of invert emulsion fluids. Invert emulsion fluids formulated with traditional diesel, mineral or the newer synthetic oils are the highest performing drilling fluids with regard to shale inhibition, borehole stability, and lubricity. Various limitations of these fluids, however, such as environmental concerns, economics, lost circulation tendencies, kick detection, and geologic evaluation concerns maintains a strong market for high performance water-based fluids. Increased environmental concerns and liabilities continue to create an industry need for water-based drilling fluids to supplement or replace the performance leading invert emulsion mud performance.
A particular problem when drilling into shale formations with drilling muds or fluids is the pore pressure increase and shale swelling from the penetration of the shale by the fluid. Shale stabilizers are typically added to the mud to inhibit these phenomena and to stabilize the shale from being affected by the mud.
Reducing drilling fluid pressure invasion into the wall of a borehole is one of the most important factors in maintaining wellbore stability. It is recognized that sufficient borehole pressure will stabilize shales to maintain the integrity of the borehole. When mud or liquid invades the shale, the pressure in the pores rises and the pressure differential between the mud column and the shale falls. With the drop in differential pressure, the shale is no longer supported and can easily break off and fall into the well bore. Likewise, the invasion of water into the shale matrix increases hydration or wetting of the partially dehydrated shale body causing it to soften and to lose its structural strength. Chemical reactivity can also lead to instability. There is always a need for a better composition and method to stabilize the shale formations.
There is an analogous need to seal and prevent fluid loss control when recovering hydrocarbons from sand formations, particularly in depleted sand formations. Depleted sand formations are productive, or formerly productive, hydrocarbon zones that have been produced, drawn down, or otherwise depleted of their content, creating a lower formation pressure than that of the fluid which may be in use in the well. Because of this pressure differential, it is important to partially or completely seal the sand formation to inhibit or prevent fluid loss of the mud into the sand.
U.S. Pat. No. 6,703,351 to Stowe, et al. (Baker Hughes Incorporated) involves a water-based drilling fluid having a polymer latex capable of providing a deformable latex film on at least a portion of a subterranean formation. This was discovered to provide reduced drilling fluid pressure invasion when used to drill in shale formations for hydrocarbon recovery operations. A precipitating agent such as a silicate or an aluminum complex (e.g. sodium aluminate) is preferably used in conjunction with the polymer. Typically, the water present contains a salt to form a brine, often to saturation, although the invention may be practiced with fresh water. If a salt is employed, it is often helpful to additionally employ a surfactant, such as a betaine, for example.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve all of the necessary characteristics for the specific end application. Because the drilling fluids are called upon to do a number of tasks simultaneously, this delicate balance is difficult to achieve.
It would be desirable if compositions and methods could be devised to aid and improve the ability of drilling fluids to simultaneously accomplish these tasks.